Ship's Lines and Model Testing

As
stated in reference (1), "The shape of a ship's hull, whether it is slim and
graceful or full and bulky, is the very essence of its character. This form
determines the power required to drive it; it reflects directly the ship's
speed; it determines the quantity of payload and the comfort and habitability
within the ship; more important, it largely establishes the limits of safety and
stability as well as the motion of the ship among waves." A ship is a three
dimensional form and in order for a naval architect to produce drawings of the
ship a conversion to the two dimensional medium is required. In the past, naval
architects were required to produce these drawings by hand, but today computers
can perform this function much more efficiently and accurately. There are three
projections used for the ship's hull, as depicted in the drawings directly above
this paragraph. They are the sheer plan, the half-breadth plan, and the body
plan. The sheer plan shows the hull from the side, the half-breadth plan from
the top, and the body plan from the front. The body plan shows the forward
sections on the right and the after sections on the left of the center line. In
order to make measurements between plans, reference lines are drawn on each
plan. The reference lines are waterlines,
sections,
and buttock
lines. The waterlines are generally spaced one to two feet apart and are
numbered from the keel. The one waterline that is not just a reference line, but
an actual design feature is the design waterline (DWL), which represents where
the ship is designed to float at a set load. The section halfway between the
first and last is known as the midship section and designated by the symbol seen
in the SNAME homepage. The first section is
called the forward
perpendicular and the aftermost one is called the after
perpendicular. The longitudinal plane splitting the ship equally and
vertically is known as the centerline plane and the line as the center line.

After the lines drawing is produced, a table of
offsets is developed and a ship model created for testing. An example of a
model testing basin is located at David Taylor Model
Basin. Model tests are run to determine the resistance
of the model, which can then be scaled up to determine the resistance
of the ship. The resistance
can then be used to determine the power required to operate the ship. The theory
behind using model tests to determine ship resistance was proposed by William
Froude and is too extensive to discuss here, but can be found in most of the
books listed in the reference
section.

Stability and Ship
Characteristics

The diagram presented above may look
confusing, but I intend to present enough information for you to learn more than
you may want about it. Many of the requirements that a ship must fulfill are
common to all types whether they are warships, passenger ships, cargo ships, or
any floating vehicle. An exception to some requirements is submarines, but I
will not discuss those topics here. The first requirement is that the ship must
satisfy stability
principles. The two forces that are required to maintain equilibrium are the
force of buoyancy
and gravity.
Buoyancy is a fundamental physical law that was defined by Archimedes in the
second century B.C. and is known as Archimedes' Principle. In the diagram above
the buoyant force acts through the center of buoyancy,point B, and as you can
see the arrows indicate an upward force. As the ship is heeled,
the position of the center of bouyancy will shift, as point B1 in the diagram
above indicates. The force of gravity acts through the center of gravity, point
G, where all of the weights of the ship may be said to be concentrated. The
other major point in determining stability is the metacenter,
point M. The key points to know for introductory stability is: (1)
M must be above G in order to be in stable equilibrium, (2) if M is equal
to G then you have neutral equilibrium, and (3) if M is below G then you
have unstable equilibrium and the ship will not remain upright and you have
failed your objective. The distance between G and M is known as GM, or metacentric
height. The keel,
point K on most diagrams, is used as a reference for measuring the other points.
A pneumonic that I have used to remember the positively stable order of the
points is MGBK, Mother Goose Beats Kids. I know this sounds bad, but I never
forget it. Vessels must also satisfy damaged stability, but that is an extensive
subject to research using the references
listed.

Ships are presented by the naval architect with given characteristics. These
characteristics are terms that all should know to be able to talk the language
of the naval architect. They include the length
overall (LOA), length
between perpendiculars (LBP or LPP), displacement,
gross
tonnage, beam,
draft,
and depth.
In addition, there are several coefficients
of form which must be understood by the naval architect, but is not required
for this simple presentation. The traditional weight term used in naval
architecture is the long ton,
as discussed in the past
section.